Immune Mediated Haemolytic Anaemia (IMHA) in Dogs

IMHA is a relatively common cause of low red blood cell count in dogs but quite a rare condition in cats.

What causes IMHA?

Red blood cells carry oxygen around the body to all cells; oxygen is essential for energy production by the cell and without it cells cannot perform their necessary functions and will eventually die. Red blood cells also carry away waste carbon dioxide back to the lungs to be breathed out.

Immune Mediated Haemolytic Anaemia (known as IMHA) is caused by the patient’s own immune system attacking the red blood cells in their body and destroying (called haemolysing) them at a faster rate than the bone marrow can make new ones. As a result, the patient becomes progressively anaemic and clinical signs will develop.

Around 60-75% of cases of IMHA appear to be primary disease in which the immune system itself is at fault. In the remaining cases, the IMHA is secondary to another disease process such as cancer or infection. In some breeds such as cocker spaniels IMHA occurs more often, suggesting a genetic component.

How do dogs with IMHA present?

In dogs, IMHA presents in a variety of ways depending on the speed with which their anaemia has developed and whether the rate of red blood cell destruction is so fast that it overwhelms the liver’s ability to metabolise the haemoglobin that is released, resulting in jaundice.

A pale and jaundiced dog with IMHA

A pale and jaundiced dog with IMHA

Patients will tend to show signs of:

  • Reduced ability to exercise
  • Lethargy
  • Weakness

Signs can be slowly developing or relatively sudden in onset. Jaundiced or severely anaemic patients may also show inappetence (lack of appetite) and depression, increased respiratory rate and effort and sometimes vomiting; severe cases may be collapsed.

How is a diagnosis of IMHA made?

Anaemia is suggested by the presenting clinical signs and physical examination revealing pale and/or jaundiced (yellow) mucous membranes. The anaemia can be confirmed by a simple blood test to measure the PCV (packed cell volume). PCV is measured by spinning the blood sample in capillary tubes so that the red blood cells separate from the fluid component. The PCV is the proportion of the column that are red blood cells (normally between 35 and 55%).

An illustration to show how PCV is measured in capillary tubes

An illustration to show how PCV is measured. The PCV of the dog with IMHA is much lower than the PCV of the normal dog.


Autoagglutination of red cells in a blood smear – typical of IMHA cases

Autoagglutination of red cells in a blood smear – typical of IMHA cases

There are many other causes of anaemia in dogs and further testing is likely to be necessary to establish a diagnosis of IMHA, including examination of a blood smear for the typical signs of IMHA such as autoagglutination (red cell sticking together) and spherocytes (red cells becoming rounded) or other pointers such as Coombs’ testing.

Investigations looking for underlying cause may involve blood tests for infectious diseases such as Ehrlichia, x- rays or ultrasound looking for mass lesions such as cancers. Results can take several days before they are available.

How is IMHA treated?

A Doberman with IMHA receiving a blood transfusion

A Doberman with IMHA receiving a blood transfusion

Treatment will depend on how unwell the patient is. Immediate life-saving therapy may be necessary if the anaemia is severe even before a diagnosis has been established. This usually involves a blood transfusion, but artificial blood products are also available.

Treatment of IMHA itself can be divided into three parts:

1) Treatment of the underlying cause if one has been found.

2) Managing the consequences of the anaemia and destruction of the red cells such as

  • hospitalisation and drugs to control nausea and vomiting
  • pain relief
  • intravenous fluid therapy
  • nutritional support

3) Drugs to directly treat the IMHA act to suppress the immune system. Virtually all cases will receive corticosteroids – usually prednisolone but some may also require other immunosuppressive drugs such as ciclosporin, azathioprine or mycophenylate mofetil.

  • Side effects with using high doses of corticosteroids are fairly common. Typically increased thirst and urination, increased appetite, panting, muscle wasting and weakness. These side effects usually subside with a reduction in the dose of prednisolone as the IMHA is brought under control. Any unusual, unexpected or severe medication side effects should be discussed with the veterinary practice. Sudden withdrawal of corticosteroid therapy can be dangerous.
  • Side effects with other immunosuppressive drugs also occur and will be discussed at the time the specific drug is prescribed by the veterinary surgeon.

What is the outlook for dogs with IMHA?

Sadly, despite intensive treatment, not all dogs with IMHA will survive, with most studies suggesting a survival rate of between 50 and 75%. A number of studies have also tried to look at the initial presentation and laboratory values in IMHA cases in order to try and predict which cases will respond well to treatment and which may not. Unfortunately few parameters are predictive of survival – for example a lower red blood cell count does not necessarily carry a worse outlook than a moderate count. The presence of other organ failure such as raised kidney values is statistically associated with a worse outcome, but on an individual patient basis this does not mean that a particular individual will survive or not.

Treatment of IMHA usually last around 12-20 weeks though some patients will relapse or require long term control of the immune system. Monitoring the patient response is an important part of successful therapy.

Key Points

  • IMHA is a relatively common disease in dogs that can cause severe and life threatening anaemia.
  • Diagnostic testing is required to establish that IMHA is the cause of the anaemia and whether there is an underlying cause.
  • Treatment may involve hospitalisation for a number of days, blood transfusion and other supportive care along with suppression of the immune system.
  • Gradual withdrawal of therapy whilst monitoring the disease is an important part of successful therapy.
  • In the initial stages, the outlook for dogs with IMHA is guarded.